To use the LLVM toolchain from Visual Studio, select a project in Solution
Explorer, open its Property Page (Alt+F7 by default), and in the "General"
section of "Configuration Properties" change "Platform Toolset" to
"LLVM-vs2012", "LLVM-vs2013", etc.

Alternatively, invoke MSBuild with /p:PlatformToolset=LLVM-vs2013
to try out the toolchain without modifying the project files.